A kinetic study on singlet molecular oxygen (O2(1Δg)) generation and quenching by dihydroxynaphthalenes

A kinetic study on the aerobic (O2(1Δg)-mediated) photooxidation of a series of dihydroxynaphthalenes was carried out. Members of this family of compounds include important environmental contaminants. The interaction with O2(1Δg), determined by both time-resolved phosphorescence and static methods, yields a quenching rate constant in the range of 105–109 M−1 s−1, depending on the solvent polarity, pH, and substitution pattern of the dihydroxynaphthalene. According to experimental evidence, the mechanism of the interaction seems to be mediated by an encounter complex with a considerable charge transfer component. From a mechanistic point of view this characteristic of DHN indicates a behaviour very similar to that of simple substituted phenols and dihydroxybenzenes. Quantum yields for DHN photooxidation (Φr) indicate that these substrates are excellent candidates for O2(1Δg)-mediated degradation under environment conditions. Furthermore, these compounds proved to be good self-sensitizers upon irradiation a...